The present invention is directed, generally, to heat transfer apparatus and, in particular, to a two-phase thermosyphon heat transfer apparatus.
In the past, heat pipe apparatus have been disclosed wherein the heat transfer fluid takes on two different phases, a vapor phase and a liquid phase. Heat transfer is accomplished using the latent heat carried by the vapor phase of the heat transfer liquid, while the liquid phase of the heat transfer liquid is utilized primarily as a means for returning the condensed vapor to the heat source. Typical of these efforts is Lazaridis, U.S. Pat. No. 3,854,454. In Lazaridis, water is heated to form a vapor, which then rises into a condenser chamber. The heated water vapor condenses on the walls of the condenser chamber thereby transferring heat from the vapor to the walls of the condenser chamber. The condenser chamber is positioned so that the condensed water is induced by gravity or a wick to flow back to the heat source portion of the heat pipe. In Lazaridis, the heat pipe is an L-shaped member with the horizontal portion being the heat source area, and the vertical portion being the condenser chamber. The heated water vapor rises from the horizontal leg and up into the condenser chamber. The cooled condensate flows down along the walls of the condenser chamber and back into the heat source area.
It is popularly believed that heat transfer in a heat pipe of this type is most efficient when heat is transferred by way of a vapor-to-liquid phase change heat transfer. In the present invention it has been discovered that heat transfer performance as high as, or better than, the apparatus of the prior art can be achieved without using the vapor-to-liquid heat transfer mechanism as the only heat transfer mechanism.
One significant drawback to using a single conduit vapor-to-liquid phase change technique as above is that condensed liquid returning to the evaporator section can be entrained by vapor flowing in the opposite direction. This can cause the evaporator to dry out and prevent effective heat transfer. To avoid this, vapor velocities must be kept low which, in turn, requires large diameter conduits.
Another drawback is that the condensed liquid which flows down the sides of the condenser chamber acts as a barrier between the heated vapor and the cooler wall of the condenser chamber. This layer of condensate has a thermal conductivity which is significantly lower than that for the wall of the condenser chamber. As such, the efficiency of the heat transfer between the vapor and the condenser chamber wall is reduced by the presence of the thick condensate layer.
Pumped-liquid loops have also often been used to transfer heat from a heat source to a heat sink, as in "side arm" domestic water heaters. These require the added expense of a pump and, in the presence of hard water, lead to scale formation on internal surfaces. Heat leaks can be significant when the device is turned off, and, upon turning off, significant amounts of heat can also be lost due to the cooling of the pump, the heat source components, and the liquid contained in the heat source components.